The present invention relates to a system and method of controlling a V-belt type continuously variable transmission (refer hereafter to as “CVT”), and more particularly, to the system and method which can prevent slippage of a V-belt from occurring when starting a prime mover of a vehicle after cruising the vehicle in a tractive manner.
With the CVT, the line pressure or source pressure is regulated through a shift control valve to obtain the hydraulic pressure to be supplied to a cylinder chamber of a primary pulley (refer hereafter to as “primary-pulley pressure”). The primary-pulley pressure is increased or decreased by the shift control valve to change the groove width of the primary pulley, changing the diameter ratio between the primary and second pulleys. With this, the shift ratio of the vehicle such as an automobile is controlled in a continuous manner. The shift control valve is connected, for example, to about the midpoint of a shift link constituting a mechanical feedback mechanism. The shift link has both ends connected to a movable flange of the primary pulley and a step motor or shift actuator, respectively. A return spring is disposed in a cylinder chamber for providing the hydraulic pressure to the secondary pulley (refer hereafter to as “secondary-pulley pressure”) to maintain a given groove width with respect to a variation and sudden reduction in secondary-pulley pressure.
Some shift control systems for carrying out shift control of the CVT initialize, during vehicle stop and at prime-mover start, a motor for urging the shift control valve to stroke to a position corresponding to a desired shift-ratio command value so as to prevent inconsistency between a motor rotated position and a motor command value at prime-mover start. In this connection, refer to U.S. Pat. No. 5,624,349.
When cruising the vehicle equipped with such CVT in a tractive manner, the shift ratio remains low since the return spring is provided to the secondary pulley. Thus, the primary pulley rotates at high speed, causing a problem of reducing the durability of parts such as a bearing. In order to prevent such inconvenience, a return spring can be provided to the primary pulley so as to change the shift ratio to the high-speed side during tractive cruising.
When the prime mover of the vehicle is started after tractive cruising, however, the shift control system for controlling shift operation of the CVT, which cannot detect the shift ratio during tractive cruising, actuates the shift actuator in the same manner as at ordinary start of the prime mover. In this case, the shift actuator operates on the assumption that the pulley ratio is close to the lowest ratio. However, with the primary pulley not located at the lowest position and the step motor located at the lowest position as the fulcrums, the shift link is positioned to make the shift control valve allow communication between a drain port and a primary port. Thus, the primary-pulley pressure is not supplied to the primary pulley until the vehicle cruises to drive the step motor in the upshift direction, causing possible occurrence of belt slippage due to downshift and insufficient primary-pulley pressure, leading to a reduction in durability of the V-belt.